Methods for preliminarily storing yarn and feeding devices structured for carrying out said method are known from EP-A-580 267 and EP-A-327 937. According to said documents, a stop signal for the electric winding drive and a selected position signal derived from the relative rotational movement between the winding element and the storage body are directly followed by a slow crawl speed-rotation in the winding direction. The winding drive first is braked electrically to said crawl speed and rotates slowly over a predetermined rotation angle into a predetermined rotational position until it finally stops. During the stop condition a tension force still can exist in the yarn between the winding element and the storage body. Alternatively, resilient components of said feeding device, e.g. an elastic filling body or resilient dust sealings, may generate a backturn torque in said winding drive. A backturn motion can result from this backturn torque by which the yarn of at least the first winding on the storage body gets loose and may wander over further yarn windings on the storage body. After a resting period the winding drive again is started, and said loose yarn suddenly gets stretched and might break. Alternatively, a loose part of the yarn laying on other windings may be forced to remain in said incorrect position from which a fabric fault within the textile machine might result which consumes the yarn from said feeding device. This is particularly critical in air jet weaving machines having feeding devices which by means of a stopping device release a yarn section of a predetermined length for each insertion cycle. The successively withdrawn windings are surveyed to actuate the stopping device shortly before the desired released yarn length is reached. In case of intermingled windings, single windings are difficult to detect during withdrawal, also leading to a fabric fault. The undesirable effect of a backturn motion of the winding drive during a resting period is disadvantageous for other types of feeding devices as well, e.g. for feeding devices of projectile or gripper weaving machines, particularly for multi-colour weaving with occasionally long lasting resting periods for one colour, irrespective of whether the feeding device is equipped with a stationary or a rotatable storage drum or with a storage drum having a fixed or a variable diameter. Said backturn motion may even amount to a circumferential stroke of the exit of the winding element between e.g. 3 and 7 cm.
JP-A-05 179538 discloses a method for preliminarily storing yarn on a drum-shaped storage body of a feeding device. According to this method a holding torque oriented in the winding direction is electrically generated within said winding drive prior to the beginning of a resting period. Said holding torque is maintained when said winding drive is in its stop condition and during the resting period. According to the known method the winding drive gradually slows down to a stop after a first harsh deceleration. The stop condition is maintained, even if tension in an upstream yarn section tends to turn the winding drive in the opposite direction. There is no controlled crawl speed rotation during a predetermined period of time or over a predetermined rotational angle since it is deemed to be sufficient to produce said holding torque prior to the true stop of the winding drive. Even if according to said method no backturning of the winding drive occurs, it cannot be excluded in practice that during stopping of the winding drive an upstream-loop formation will occur in the yarn and/or the winding drive will experience after run due to the early effect of the holding torque.
It is an object of the invention to provide a method of the kind as disclosed and a feeding device for carrying out said method, which avoid disturbances and damages due to a backturn motion after a stop.
Said object can be achieved by generating a holding torque within the winding drive and in the winding direction during a resting period to prevent rotation of the winding drive.
The crawl speed rotation of the winding drive during a predetermined period of time or over a predetermined rotational angle, which is intentionally controlled prior to switching on said holding torque, servres to correctly control the yarn in the run out phase and optionally also to bring the winding drive finally to a standstill at a predetermined position. The holding torque switched on first at standstill prevents relaxation or loosening of the yarn during the resting period. Since said holding torque is maintained during the resting period, the proper yarn positioning cannot change during the resting period. At a new start of the winding drive after a resting period there is no danger of a yarn breakage or a fabric fault due to a loose yarn or a winding which has fallen with respect to other windings. Said holding torque acts in addition to the system dependent, predetermined rotational resistance of the winding drive and the components coupled therewith, however, without generating any rotation in the winding direction. The winding drive, so to speak, is statically pre-biased in the winding direction after first having carried out said crawl speed rotation.
The use of the feeding device significantly reduces the quota of yarn breakages and fabric faults. This is of particular advantage in weaving machines in which pattern depending selected feeding devices have to stop for longer resting periods. The backturn detent system can be actuated by a holding current with exact timing and is, therefore, even sufficiently precise for fast running feeding devices in order to then suppress an undesired backturning. For said additional function components may be used which already are provided for control and for function purposes of the feeding device.
The holding torque is adjusted with constant magnitude and is simply controlled by a holding current or a holding voltage, respectively. The holding torque is switched on as soon as a resting period starts and is maintained during the entire resting period.
The holding torque is switched on prior to, after, or exactly at the stop. It would be ideal to switch on the holding torque shortly after the stop, e.g. some milliseconds later, in order to have a co-operation of the mechanical, friction depending starting torque of the winding drive which for physical reasons first occurs in both rotational directions with the actual stop condition. However, this may be complicated to control, as the precise point in time of the mechanical stop of the feeding device must be detected and then the generation of the holding torque must be matched therewith.
In one embodiment holding torque is adjusted depending upon the yarn quality and/or mechanical rotational resistance. It is adjusted to completely or at least largely compensate for the expected backturn torque, but not to cause further rotation in the winding direction.
The frequency is raised by a multiple while simultaneously the voltage is lowered in order to avoid a step function when switching on the holding torque. Said measure allows one to adjust and maintain the holding torque precisely.
The holding torque is controlled by a microprocessor of the electrical control device. Said microprocessor is prepared at its software side for said task. Microprocessors and control electronics as usually employed in feeding devices are capable of fulfilling said additional task without the need for structural modifications of the feeding device.
Even with the holding torque, under certain operating conditions occasionally a slight backturn motion or a slight rotation in the winding direction might occur. However, this is tolerable within the frame of the solution of the task of the invention, since the operational safety already is significantly improved when backturning is avoided with substantial reliability or does occur only seldom and to a lesser extent.